In the context of this invention the term “document of value” refers to bank notes, checks, share certificates, tokens, ID documents, credit cards, passports and other documents as well as labels, seals, packagings or other elements for the product protection.
Protecting documents of value against forgery by means of luminescent substances has already been known for a long time. The use of transition metals and rare earth metals as luminescent ions has already been discussed. Such ions have the advantage that they, after appropriate excitation, show one or a plurality of characteristic narrow-band luminescences which facilitate a reliable detection and the delimitation against other spectra. Combinations of transition metals and/or rare earth metals have also been already discussed. Such substances have the advantage that, in addition to the above-mentioned luminescences, so-called energy transfer processes are observed which can lead to more complicated spectra. In such energy transfer processes an ion can transfer its energy to another ion and then the spectra can consist of a plurality of narrow-band lines which are characteristic for the two ions.
But ions with characteristic properties, which are suitable for protecting documents of value, are limited in their number. Moreover, the ions of the transition metals and/or rare earth metals luminesce at one or a plurality of characteristic wavelengths which are dependent on the nature of the ion and of the host lattice and can be predicted. Energy transfer processes, too, lead to these characteristic luminescences of the involved ions.
DE 198 04 021 A1 describes a document of value with at least one authenticity feature in the form of a luminescent substance on the basis of doped host lattices.
EP 1 370 424 B1 describes a printed document of value having at least one authenticity feature in the form of a luminescent substance on the basis of host lattices which are doped with ions of the (3d)2 electron configuration.